A group of teenage skateboarding and bike-riding friends were relaxing and chatting on a shady patio at Sunvalley Mall in Concord.

But if, say, a parent or other adult were tempted to eavesdrop, they'd need a translator.

Although someone older than 18 might know that "chillin' " means relaxing, they'd probably be lost when it comes to "sick" (a difficult skateboard trick) or "twomps" (a $20 bag of marijuana).

The teenagers had not invented a language, of course. They were simply using the vernacular of their peers.

What they were doing, say language experts, was identifying themselves as young people, playfully experimenting with their language while trying to discover who they are. The parental confusion and exclusion is probably intentional.

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Berkeley lawyer and slang expert Tom Dalzell, who has written two books on slang, "Flappers 2 Rappers" (Merriam-Webster, 1996) and "The Slang of Sin" (Merriam-Webster, 1998), said most experimentation with language occurs during the junior high and high school years.

"There's a lot more socializing in those years," he said. "There's feelings of rebellion. You're creating identities during adolescence, and you feel part of a language that bonds you. You feel alienated, and it bonds you with others who feel alienated. Slang has a way of marking you - 'I'm a young person.' "

Dalzell is also senior editor of the New Partridge Dictionary of Slang, a two-volume, 2,000-page dictionary to be published in 2005 by London publisher Routledge.

The language seems as fickle as it is steadfast: Cities may have slightly different slang words - especially for things like drugs - but, for the most part, most popular slang is the same.

Antioch dental assistant Kay Wall, 47, said although her son, Matt, 22, used a great deal of slang when he was a teen, she was always able to communicate clearly with him, even when he seemed to be talking like one of his rap albums.

"I never hesitated to ask him," she said. "I tried to keep up, but I didn't use it - that would embarrass him. I tried to know the words and understand how they're used, just to know what they're talking about."

Wall's 15-year-old neph-ew, Craig Frazier, who was visiting from Portland, Ore., had a bit of advice for parents struggling to understand their children: "Parents should always listen to their kids and what they have to say. Some parents don't listen - they just turn their backs, and their kids can say whatever they want. It makes me mad. They could learn a lot."

Wall said her son eventually phased out most of his slang words.

"He talks much clearer ever since he got away from rap music. He got into the blues," she said. "He has much more variety now. His English got better, he learned to speak better, learned how to talk better. So it does change."

In many cases, young men seem to use slang far more often than young women.

The mother and daughter were shopping at Northgate Mall recently with their next-door neighbor, 10-year-old Alex Maffei. Both girls said they do not use slang.

"They speak a different language, and I think the girls are becoming teenage girls, so you can see a difference in maturity. And as for the boys," Rose Martinez said with a laugh, "I don't know about them sometimes."

The sources of slang words vary: Some may have carried over from years or decades past, while others may have been introduced by new movies and music videos, and still others may have been invented by a particular group, and may last only a short time.

A few months ago, for example, some San Rafael teen athletes coined a term "ink," which meant to party. The word was popular for a few weeks and disappeared.

"As a species, we are almost hard-wired to play with language," said Dalzell, the slang expert.

Much of the slang favored by the San Rafael teens was by no means new to the slang world: "Sick" is a common term among board-sport athletes, "twomps" appears to be common in drug circles, and "chillin' " has been around so long it is surprising that it has not become obsolete.

"You just kind of pick it up," said Nik Mihalko, a sophomore at Terra Linda High School. "You get it from people you meet. It doesn't go through your mind like, 'I met this person, I'm gonna to pick up this word.' But if you meet people, you might just start talking like them."

As 16-year-old Andrew Smith of Oakland shopped with three of his friends for a new video game in a Funcoland game store in Emeryville, he described one of his favorite games with what may be the most popular term used today: "tight."

The word describes a situation or a product that is exceptionally good or appealing - what many parents would have likely called "groovy" during their adolescence.

According to Dalzell, use of "tight" has come full circle since its appearance about 50 years ago.

Dalzell said jazz musicians in the 1950s and early 1960s used the word "uptight" to describe something that was very good; then, in the mid- to late- 1960s, hippies used "uptight" - to describe someone who was inhibited or overly cautious; now, the shortened version means exactly the opposite.

"It's almost like these words are notional, or have lives of their own - they sit around plotting how they will come back," Dalzell said. "Of course, that doesn't happen, but it happens."

Dalzell said most slang words have roots in African American culture.

"It serves two functions," he said. "It bonds the people who are oppressed, and it functions as a gesture of resistance. You think about slang in America since 1935. To a large degree, larger than most people realize, slang has developed from African American slang - the most oppressed group in America.

"If you also look at gays, look at prisoners, look at boarding-school students, look at enlisted men in the Army, there are different levels of oppression, to be sure, but there is great slang."

Another slang term, "hip hop," has come full circle.

According the Dalzell, "hip hop" originated in the early 1900s from African Americans' use of "hip." In the '40s, the word became "hipster" to describe those in the jazz scene. In the '50s, "hippie" again described someone in the jazz scene, then in the 1960s the flower children. Now "hip hop" signifies the rap music scene.

"That's every generation sort of coming along and making their own mark on the language," Dalzell said. "So, you see, it is almost imperative to play with the language, and that is happening an awful lot when people are moving through junior high school and high school."

Video game-shopper Smith, a junior at Oakland Tech High School, said using slang doesn't cause communication problems at home.

"I live with my grandma, and she pretty much understands everything I say with slang, because I've been talking like that since I could talk," he said.

It's a different story, however, when Smith's parents come around.

"When I'm with my parents, I don't use too much slang," he said. "I think it's kind of disrespectful to use slang if they don't understand what you're saying."

In March, Berkeley High School English teacher Rick Ayers helped clue in many adults as to what kids were saying.

With the help of students from his Communication Arts and Sciences class, Ayers produced the 200-word Berkeley High Slang Dictionary, which provides definitions for words such as "schmabbin' " (driving fast, or driving around with a group of friends) and "gaffle" (to steal).

Four-hundred copies of the book have been sold, and many more have been given away.

Ayers noted that two topics in particular - marijuana and friendship - each generated a large amount of slang. The dictionary lists more than 20 words for each.

Ayers noted that teens sometimes make up words simply because they rhyme with existing slang words.

For example, "off the hook," used to describe a good situation, became seriously outdated and was replaced with a modified, "off the heazy." For further emphasis, some teens modified the phrase, adding "fo' sheazy," (for sure) to the end of it - "off the heazy, fo' sheazy."

"I think (slang) is an essential part of their identity," Ayers said. "They are inventing their identity. They decide one year to the next - I want to be a Goth, I want to be into hip hop - and all this language stuff is playing with that. It's such an important time in their lives, there are a lot of changes, and that's where slang comes in."

But there is a trend that Ayers said he finds disturbing: Some teens are using the word "gay" to describe something they feel is "stupid," and "ghetto" - which Ayers said is a coded word that negatively refers to African Americans - to describe something that is run-down or trashed.

There are other examples that refer to women and other minorities.

Ayers said when he hears such uses he tries to educate his students about the importance of using slang without offending anyone.

"I don't want to be the word police - I do with the word 'gay.' But with ghetto, it's not about saying, 'Don't say that,' it's more about trying to raise their consciousness," he said.

"If you see something stupid, you say it's stupid, you don't say it's gay," he said. "It's just like you wouldn't go up to a black person and call them the n-word. You've got to use your head before you say something."

Pacheco, who uses his stage name when playing bass in his band Me-Crab, was hanging out at the Tanforan Park Mall in San Bruno on a recent afternoon with two friends from San Bruno, Jeska Trujillo, 15, and Sarah Nye, 14.

Slang use, Pacheco said, comes second nature to most kids because they are surrounded by it.

As the group discussed its favorite punk bands - Blink 182, NoFX and Weezer - Jeska acknowledged that music plays a big role in how teens use slang.

She said she and her friends might use different words to describe something than, say, a group of kids who listen exclusively to hip-hop.

"If you listen to it all the time, and you're around people who talk like that, you're going to do it," she said."

So, what's a parent to do if they want to understand what their kids are saying?

Either don't try to be like your kids, or tell your kids to speak regular English around you, and everything will be all right, because they'll never catch up to us."

Dalzell, however, has a different suggestion.

"Take the language seriously, and ask your kids what it means," he said. "Because by the time you get it written down, it's dead, almost by definition."

What it is

For parents confused by what their children are saying, check out the rap dictionary online at www.rapdict.org.

Slang-English dictionary

An online Teen Slang Dictionary sponsored by About The Human Internet contains terms that can bring your vocabulary from the dark ages of being "cool" to today's being "all that." Here are a few of entries: